The present disclosure is related generally to the field of combustion. More particularly, the present disclosure is related to combustion air control.
Boilers can be equipped with air control systems. The amount of air supplied to a combustion chamber can determine the amount of pollutants, such as carbon monoxide (CO) and nitrous oxide (NOx), among other pollutants, formed during combustion of fuel in the boiler and also the efficiency of the boiler.
In boilers, a minimum amount of air is necessary to completely burn a fossil fuel. The stoichiometric amount of air to completely burn a fossil fuel is the amount of air that contains exactly the number of oxygen molecules necessary to oxidize the fossil fuel completely.
A boiler with less than the stoichiometric amount of air can result in incomplete combustion of the fossil fuel, which leads to inefficient boiler operation. A boiler with more than the stoichiometric amount of air can avoid incomplete combustion of the fossil fuel, but may cause more energy to be lost in the stack of the boiler.
The energy lost due to excess air in the boiler can be called stack and/or sensible heat loss and is caused by using energy to heat the extra air during combustion which therefore is not used to create steam during the combustion process. Accordingly, stack heat loss also causes inefficient boiler operation.
The amount of air above stoichiometric air that provides an acceptable balance of the losses associated with unburned fossil fuel and stack heat loss can be called the excess air fraction. The excess air fraction can be dependent on many factors, such as boiler construction, air-fuel mixture homogeneity, fuel type, and boiler size, among other factors.
The excess air fraction can be related to the oxygen concentration in the flue gas of a boiler. An oxygen concentration of approximately 2% in the flue gas of a boiler can indicate approximately 10% of excess air. In some boilers, 10% of excess air may be utilized to minimize the total losses associated with unburned fossil fuel and stack heat loss.
Some boilers can include an air feedback control system to adjust the amount of air added to the combustion chamber of the boiler, so the suitable amount of excess air in the boiler can be maintained. For example, if less oxygen is sensed in the flue gas, the air to fuel ratio can be increased until the suitable amount of oxygen (e.g., 2%) is sensed in the flue gas.
However, controlling the exact amount of air in the combustion chamber may not be easily accomplished. Some boilers are operated at low pressure to prevent poisonous gases from escaping during the combustion process.
This lower pressure allows air to enter the boiler through gaps in and/or between various components of the boiler. The amount of air that enters the boiler uncontrolled may be referred to as leaking air. Some air feedback control systems lack the ability to compensate for leaking air during transients, e.g., when changing the power level of the boiler. when adjusting the air to fuel ratio to provide the suitable amount of excess air for the boiler and, therefore, may result in a loss of efficiency.